In recent years, a flat panel detector (FPD) which includes fluorescent substances disposed on a TFT active matrix substrate, which accumulates X-rays as charge, and which provides X-ray image data by converting the charge into digital signals has been in practical use and frequently used. Such an X-ray imaging apparatus is integrally formed with general modality and configured as a dedicated apparatus. However, there is a demand for replacing an imaging medium, such as a film and an imaging plate, of existing modality by the FPD.
In a case where the imaging medium of the existing modality is replaced by the FPD, construction of an interface between an X-ray generation apparatus and the FPD may be difficult. To address this problem, PTL 1 discloses a technique of starting an operation of accumulating charges by detecting irradiation with X-rays on an FPD side without an interface between an X-ray generation apparatus and an FPD.
Here, although the FPD converts photons into electrons by photodiodes, dark charge is generated due to thermal excitation or leakage current even in a case where photons do not exist in practice. When the dark charge is accumulated, the dark charge is added to charge generated by X-rays actually emitted, and therefore, non-uniformity of data is generated resulting in generation of artifact. Furthermore, since the dark charge is accumulated, an amount of the accumulated X-rays is reduced. Therefore, the FPD performs a resetting operation so that the dark charge is discharged while TFTs are constantly in an on state before X-ray imaging is performed.
However, in a case where the operation of accumulating charge by detecting irradiation with X-rays as described above is started, a certain amount of X-rays is required before the charge accumulation operation is started, and the resetting operation is performed before the charge accumulation operation is started. Therefore, the charge generated by the X-rays emitted before the charge accumulation operation is started is discharged, and accordingly, data lack occurs resulting in generation of artifact.
On the other hand, PTL 2 discloses a technique of a resetting operation performed for each line at a time of detection of X-rays while an even-numbered line and an odd numbered line are replaced by each other for each frame. In the technique disclosed in PTL 2, artifact is generated every other line due to data lack in a period of time from when irradiation with X-rays is performed to when a charge accumulation operation is started, and therefore, the artifact may be corrected by an interpolation process.
Here, in X-ray imaging, imaging using a grid is frequently performed. The grid which is one of basic tools to be used in X-ray imaging shields X-rays scattered in an inside of a subject so that a blur of X-ray image data is suppressed. In imaging using a grid, in the technique disclosed in PTL 2, in a case where a direction of grid lines and a direction of scanning lines match each other, grid stripes and artifact generated at a time of X-ray detection interfere with each other. If the X-ray image data is corrected by the interpolation process, artifact may be additionally generated or the grid stripes may be partially removed.
PTL 3 discloses a technique of removing grid stripes by model fitting. However, artifact generated at a time of X-ray detection is not taken into consideration in the technique disclosed in PTL 3, and therefore, the grid stripes may not be appropriately removed in a case where the grid stripes are partially removed.